1. Field of the Invention
This invention relates generally to methods and apparatus for plotting data, whether permanently or transitorily, and more particularly to methods and apparatus for plotting a sequence of data values at a suitable scale expansion even when the largest of the values is not known in advance.
2. Prior Art
Known devices and methods for displaying and recording plots of data-value sequences are encumbered by the need to select suitable scale expansions or ranges for such plots. By "suitable scale expansion" I mean a scale expansion that presents the maximum data value, or perhaps several different data values of interest, at a point relatively near full-scale.
This constraint is important because the geometric uncertainty introduced by a plotting apparatus in positioning of the indicia on the plot, and also the user's uncertainty in visually reading the plot, are generally a constant fraction of full-scale, independent of scale position By selecting a scale expansion that places data values of interest near full-scale, the equipment operator ensures that such uncertainties will represent the smallest possible fractional error in the data value as read from the plot.
This requirement for scale selection, however, poses at least three major disadvantages.
First, complexity and correspondingly added cost are introduced into the apparatus, since a scale-expansion control and readout (sometimes combined) must generally be provided for the operator's use.
Concededly, in some bare-bones systems (especially digitally controlled ones) the scale-expansion control may be a control keyboard that is shared with other functions of the apparatus; similarly the scale-expansion readout may be a display that is shared with other functions of the apparatus. It might be argued that in such systems the complexity and cost added on account of the scale-expansion control and readout are negligible. As is well known to anyone who operates measuring instrumentation, however, such apparent simplification and economy in the apparatus are false, since they are obtained at the severe expense of introducing complexity and delay into the operator's life.
At the other end of the spectrum of equipment complexity, perhaps not often encountered in the most modern equipment, is the analog recording system in which different scale expansions are implemented by voltage-divider strings of precision resistors or by other elaborations of the apparatus itself. In this type of equipment the provision of selectable scale expansions is clearly a very significant expense.
A second major disadvantage of the requirement for scale selection arises from the limited capacity for prescience on the part of most operators. Often the maximum value to be received in a sequence of data values is not known until the sequence is complete. Therefore the operator can only make the best educated guess of the anticipated maximum value, and set the scale expansion in accordance with this guess.
If the guess is high, then the maximum data value may be displayed at a needlessly low position on the scale--resulting in excessive error due to plotting and reading uncertainties, as explained above. If the guess is low, then the maximum value will be off-scale--resulting in either complete loss of detailed information at the maximum data value, or in the necessity for resetting the scale expansion during the plotting process.
This latter possibility, the resetting of the scale expansion during plotting, has in turn certain drawbacks of its own. The operator must be vigilant to detect the need for resetting before the plot goes off-scale. The operator must take attention and the use of his or her hands away from other activities to do the actual resetting. In the case of permanent recordings the resetting introduces into the graph a discontinuity which may be esthetically objectionable if the record is to be published.
The third major disadvantage of the requirement for scale selection is that some data values of very great interest may be much smaller than the maximum value. No single scale expansion can both (1) display the maximum value as an on-scale point and (2) display much smaller values near full-scale for minimal error of positioning and reading
This basic fact of life can be a very serious aggravation with some types of data, as it can create a need for multiple resettings of the scale expansion during the progress of the data sequence, and multiple discontinuities in permanent recordings of the plot.
Some equipment designers have attempted to alleviate the second and third major problems discussed above, by providing automatic range changing as the data stream goes off-scale. As is painfully well known, however, automatic range changing introduces its own annoyances. In some types of equipment there is often an associated additional equipment expense. Furthermore, the automatic range change somehow always seems to cut in just before a peak or trough in the plot, aggravating the esthetic problem (and even the readability) mentioned above. Finally, the provision of overlapping range-change regions--sometimes used in an effort to reduce the number of unesthetic discontinuities in the graph--all too often instead only results in badly confusing the readout, and even the operator.
Prior-art techniques thus fail to satisfy the clear demand for a simple, economical way of transitorily displaying and permanently recording data of interest near full-scale, when (1) maximum values are not known at the time of equipment manufacture--or even at the time of starting the plot--or when (2) data values of great interest are substantially smaller than the maximum values in the data stream. What the prior-art fails to provide is a data plotter that has universal scale expansion, or, more simply put, universal gain.